1. Field of the Invention
The present invention relates to anti-sway devices for inhibiting and arresting sway of a suspended load and more particularly it relates to a variable centers rope suspension system for a gantry crane. The invention is particularly adapted to large dockside cranes used to load and unload standardized cargo containers.
A problem associated with the handling of cargo by large cranes is sway of the load as the load is moved between its pickup point and its deposition point. Due to the length of the ropes supporting the load, and the distance and speed with which the load must be moved to obtain the maximum utilization of the crane, the load experiences pendulum motion due to the combined action of the horizontal acceleration and deceleration forces acting on the inertia of the load.
It is necessary to accurately position the suspended load for deposition, and to effect this it is very important to be able to stop the horizontal movement of the load and accurately place and release the load in the shortest possible time in order to reduce the cycle time of the crane in effecting the transfer of each cargo container.
In a crane suspending a load from a high gantry, inhibiting sway of the load is very difficult. The load is suspended from long ropes, and the amplitude of sway is likewise greatly increased. Since a large crane is a very expensive item to both acquire and operate, it therefore is very important to obtain maximum efficiency and utilization of the crane. To do this, it is necessary to pick up and move the load as fast as possible from its pickup point to its point of deposition. Increasing the movement speed over the same length of travel intensifies the sway problem since larger horizontal acceleration and deceleration rates are produced and these are the primary causative factors inducing sway in a load.
In the very large and expensive container cranes now being utilized, where the gantry is typically 75 to 80 feet above the dock area, a need exists for an anti-sway device for high lift cranes which can increase the handling capacity of the crane by decreasing the cycle time of moving a container between the load pickup and deposition points. This can be most effectively achieved by reducing the sway of the load while it is being handled by the crane to minimize the time required to accurately position the load for set down and release.